Question: I have a Valencia seedless orange tree that I set out about 40 years ago. The fruit has always been sweet and juicy. But the past couple of years, the oranges are larger with thicker skin, not much meat and less juice. Can you tell me what I am doing wrong? — Charles Johnson, North Hills

Answer: In all likelihood, you are seeing sour fruit from a branch or branches that have grown off the bottom of the trunk.

Nearly all fruit trees, at least those available in the nursery trade, consist of a scion (desired fruit-bearing variety) grafted onto a rootstock species. The fruit you describe is from a rootstock species known as sour or bitter orange (Citrus aurantium), which was commonly used 40 years ago as a rootstock for sweet oranges (Citrus sinensis) such as Valencia and navel.

Scion varieties benefit from being grafted onto rootstock species by demonstrating qualities they would otherwise lack if clonally propagated from their own stem or shoot tip cuttings, including greater vigor and productivity, disease resistance, drought tolerance, cold tolerance and salt tolerance.

Sometimes a shoot below the graft union, emanating from the rootstock, develops into a branch or branches and begins to fruit, which seems to have happened in your case. Follow this fruiting wood back to the trunk and remove it with a saw.

Your lack of sweet oranges could indicate rootstock growth overtaking and sapping vigor from the scion, or it might simply be a consequence of getting old. Seedless Valencias may decline significantly in productivity around age 40. (Note: The average lifespan of all citrus trees is around 50 years.)

Rootstock species are chosen for the vigor they impart to the scion. However, once the productive lifespan of the scion is exhausted, the rootstock species may still be going strong and is that much likelier to take over the growth of the tree.

Hygiene is also crucial in control of this disease. You need to remove infected flower panicles at the first sign of mildew and immediately dispose of fallen panicles, leaves and fruit, since they carry spores of the fungus that may splash (during irrigation or rain) or float (from wind) back up into the tree.